1. Field of the Invention
This invention relates to a homogenous immunoassay method and a system for determining specific antibody levels to a multiplicity of allergens from a blood sample, or for determining total immunoglobulins E levels in such a sample, for the purpose of diagnosing allergy.
2. Background and Prior Art
“Allergy” is synonymous with atopy or hypersensitivity and is the result of an immunologically mediated reaction by individuals to various antigenic materials, known as allergens. People with allergies produce allergen-specific immunoglobulins, IgE and IgG, in response to exposure to normally harmless substances from pollens, molds, dander or foods, which are inhaled or ingested. The generated antibodies are released to circulate in the blood and eventually fix to specific cells in tissue. Exposure to allergens generally results in immediate or delayed reactions, manifested in a number of commonly identifiable symptoms, such as sneezing, itchy eyes, runny nose and inflammation of the lungs and nasal passages. The term “allergy” is also generally synonymous with hay fever, rhinitis, eczema, hives, and linked to the onset of asthma. The diagnosis of allergy involves a review of the patient history, physical examinations and running a confirmatory diagnostic test to identify whether the patient's symptoms are of allergic or non-allergic origin. If allergy is responsible for the symptoms, then the allergens responsible must be identified. Patients with atopic or allergic diseases may be mono-sensitive to one allergen; however, sensitization to multiple allergens is more usual. Reactions of persons to allergens can range from the annoying to the severe or even fatal. It therefore is desirable to be able to determine not only whether a person has allergies, but if so, to what allergens and to what level of severity, so that exposure can be avoided, minimized or mitigated through pharmacotherapeutic or immunotherapeutic methods
Confirmatory diagnostic testing may be conducted by in-vivo skin testing, in-vivo provocation testing, or in-vitro testing for the presence of circulating allergen-specific antibodies from blood samples. Direct provocation, by direct inhalation or ingestion of possible offending allergens, while relevant, is unpleasant, possibly dangerous and cannot be performed for multiple allergens at one sitting.
Skin testing (also referred to as skin prick testing or scratch testing) is an in vivo procedure that involves applying an allergen sample, or more generally a multiplicity of allergens, directly to a patient's forearm or back via a small needle scratch and measuring the size of the inflammatory reaction (wheal) at the applied site on the skin. Skin prick testing is widely used, is reliable under optimal testing conditions, can be painful, is subject to large differences in technique and interpretations, and cannot be used on patients taking certain drugs or patients with skin problems. Furthermore, both provocation and skin prick in-vivo diagnostic methods have the potential for sensitizing patients to new allergens and, in extreme cases, eliciting a life-threatening anaphylactic reaction upon direct exposure to the offending allergen(s).
In vitro diagnostic testing methods directly measure circulating levels of allergen-specific antibodies in a sample of blood obtained from patients. These methods are generally immunoassay procedures that are reproducible, are equivalent in sensitivity and specificity to well conducted skin prick tests, are unaffected by any of the factors which prevent the use of either of the two in vivo methods, and do not cause anaphylactic events. Immunoassay techniques capable of measuring specific antibody levels to single allergens have been employed for many years (Johansson, S. G. O. and Yman, L., In vitro assay for immunoglobulin E, Coin. Rev. Allergy 6, 93-139, 1988). Alternatively, methods that measure allergen-specific levels to a plurality of allergens simultaneously have provided more useful screenings of allergy as, for example, described in U.S. Pat. Nos. 4,459,360 and 5,082,768. U.S. Pat. No. 6,087,188 describes a method of detecting an antibody in a sample using paramagnetic particles and a chemiluminescent acridinium compound bound to avidin or streptavidin. The method described in this patent is stated to be useful for the detection of allergens. However, the method is limited to detection of a single allergen in a given sample.
In the field of clinical diagnostics there is a broad category of methods available for determining an expanding list of clinically relevant analytes. One such category is immunoassays, which are currently used to determine the presence or concentration of various analytes in biological samples, both conveniently and reliably (The Immunoassay Handbook, edited by David Wild, M Stockton Press, 1994). Immunoassays utilize specific binding agents to target analytes in fluids, where at least one such binding agent is generally labeled with a variety of compounds, including radioisotopes, enzymes and fluorescent or chemiluminescent compounds, that can be measured by radioactive disintegrations, enzymatic induced color-producing substrates, fluorescent output or inhibition and chemiluminescent light output. Such specific binding agents typically include analyte specific antibodies (immunoglobulins) and antibody fragments, receptors, lectins, and genetically or chemically engineered artificial antibodies. Notable immunoassay methods include, for example, radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELIZA) (Enzyme-Immunoassay, Edward T. Maggio, CRC Press, 1980), fluorescent immunoassay (FIA) and chemiluminescent assays (CLA) (Luminescent Assays, Perspectives in Endocrinology and Clinical Chemistry, Vol. 1, Mario Serio and Mario Pazzagli, Raven Press, 1982), (Bioluminescence and Chemiluminescense, Basic Chemistry and Analytical Applications, Marlene, A. DeLuca and William D. McElroy, Academic Press, 1981), (Journal of Bioluminescence, Vol. 4, M. Pazzagli, et al., Proceedings of the Vth International Symposium on Bioluminescence and Chemiluminescence, Wiley, 1989), etc. Numerous method variations and devices for performing such assays are available, are known to those familiar with the art, and can be found in the scientific and patent literature.
Immunoassays may be heterogeneous or homogeneous. Heterogeneous immunoassays have been applied to both small and large molecular weight analytes and require separation of bound materials (to be detected or determined) from free materials (which may interfere with that determination). Heterogeneous immunoassays may comprise an antibody or an antigen immobilized on solid surfaces such as plastic microtiter plates, beads, tubes, or the like or on membrane sheets, chips and pieces of glass, nylon, cellulose or the like (Immobilized Enzymes, Antigens, Antibodies, and Peptides, Howard H. Weetall, Marcel Dekker, Inc., 1975). In heterogeneous immunoassays, antigen-antibody complexes bound to the solid phase are separated from unreacted and non-specific analyte in solution, generally by centrifugation, filtration, precipitation, magnetic separation or aspiration of fluids from solid phases, followed by repeated washing of the solid phase bound antigen-antibody complex. Of particular interest are immunometric “sandwich” assays (Immunochemistry of Solid-Phase Immunoassay, John E. Butler, CRC Press, 1991) which first require binding of an immobilized antigen or antibody with the target analyte from the biological sample. Separation of the immobilized pair and subsequent repeated washing is followed by the introduction of a secondary binding agent specific to the analyte, said secondary binding agents usually being chemically conjugated with radioisotopes, enzyme, fluorescent or chemiluminescent labels described earlier. Secondary binding agents are typically immunoglobulin antibodies, antibody fragments, monoclonal antibodies or recombinant antibodies. The analyte is “sandwiched” between the first immobilized antigen or antibody and the labeled secondary binding agent. A subsequent separation and washing is required to remove unbound labeled secondary binding agents. Direct measurement of the labeled, immobilized bound complex or indirect measurement with the use of substrates is then undertaken. It can be appreciated by those familiar with the art of conducting solid phase immunoassays that the procedures are laborious, time consuming and require special equipment or devices for separating immobilized binding agents and analytes.
Homogeneous assays are, in general, liquid phase procedures that do not utilize antigens or antibodies that are immobilized on solid materials. Separation and washing steps are not required. The procedures are more commonly involved with the use of fluorescently-labeled antigens or antibodies which upon binding with a target analyte undergo an excitation or quenching of fluorescence emissions, due to the close steric proximity of the binding pair. Homogeneous methods have typically been developed for the detection of haptens, small molecules, such as drugs, hormones and peptides. Macromolecule analytes, such as proteins or peptides with greater than 5000 molecular weight, usually are not determined by homogeneous methods due to a lack of assay sensitivities. A homogeneous method for detection of proteins was reported in U.S. Pat. No. 5,807,675 which required chemical modifications to both binding agents, but is limited to the detection of single analytes.
For the diagnosis of allergy determination of total immunoglobulin E levels is helpful, but more importantly there is a need for a convenient and reliable immunoassay method that simultaneously measures specific immunoglobulin antibody levels to a panel of allergens, where one or more allergens may be responsible for the onset of allergic symptoms. It is further desired that the immunoassay protocol assay be easily carried out and adaptable to automation. In vitro methods currently available utilize heterogeneous immunoassay methods where separation and washing is required, making them labor-intensive, time-intensive and difficult to automate. It also would be advantageous to combine the versatility and sensitivity of solid phase heterogeneous assay methods with the ease of a homogeneous protocol. Furthermore, conducting in vitro allergy tests on a panel of allergens requires drawing a significant sample, generally 3-5 milliliters, of venous blood from the patient. Indeed, the patient must visit a laboratory or physician's office for the single purpose of having the blood sample drawn.
Recent advances in immunoassay methods have introduced microtechniques that utilize smaller solid phases and smaller sample requirements. One example is a microimmunoassay method for conducting analysis and detection of multiple biomolecules that is described in U.S. Pat. No. 5,981,180. Apparatus of this general type has been marketed by the Luminex Corporation under the trademark FlowMetrix®. The technology incorporates a flow cytometric procedure and the use of small, 5.6 micrometer polystyrene bead sets, each set containing an internal fluorescent signature, that enables detection of multiple analytes. For in vitro allergy testing, where specific levels of antibody are in low concentrations in circulating blood, undiluted blood or serum is required to enable the detection of antibodies that are at sub-nanogram- to picogram-per-milliliter levels. Up to now, heterogeneous rather than homogeneous immunoassay methods have been employed with existing in-vitro allergy testing methods, since undiluted blood or serum very often contains microgram levels of free or non-specific (total) antibody levels which can interfere with determinations using homogeneous assays.
Homogeneous assay methods using undiluted blood or serum to measure sub-nanogram- or picogram-per-milliliter levels of antibody have been known to show falsely low or falsely undetectable levels of specific antibodies in samples where the free or non-specific antibody levels in undiluted serum are in the microgram per milliliter range. Such false results are known as a “hook effect”. The hook effect is described by Robard, D., (Radioisotopes: 37, (10), 1988), in the Immunoassay Handbook (Edited by David Wild, M Stockton Press 1994) and the Manual of Clinical Laboratory Immunology (4th ed., Rose, et al., Editors) published by the American Society for Microbiology (Chapter 2, page 5). It involves an unexpected fall in the amount of an analyte at the high end of a dose-response curve, which results in a gross underestimation of the analyte. The hook effect is caused by high concentrations of free, i.e., unbound, analyte from neat serum or blood samples that bind to secondary binding agents, depleting the availability of the secondary binding agent to the solid phase bound analyte, subsequently rendering a falsely lower signal, hence indicating a falsely low analyte concentration. Heterogeneous assay formats, specifically, immunometric sandwich assays, generally circumvent the interference with the separation and washing steps inherent to the procedures.
The above-mentioned references suggest several ways that laboratories can deal with this hook effect. One suggested strategy is to run all patient samples at two dilutions as a screen for this problem. If the more dilute sample indicates a significantly higher level of analyte, the laboratory is alerted to the possibility of a hook effect. Various dilutions can then be carried out to provide an accurate determination of the existing amount of analyte in the sample. Such procedures, of course, result in duplication of work and lengthening of the time and costs required to conduct testing of the sample from a given patient. Alternatively, large excesses of secondary binding agent, sufficient to bind both bound and unbound specific and non-specific analyte antibody can be used. However, this approach has not proved practical due to the large concentrations of unbound specific and non-specific analyte antibody in undiluted blood, the relatively large serum or plasma volumes typically required in conducting in-vitro allergy tests, and the costs for secondary binding agent that would be used.
It would be advantageous to provide a homogeneous assay method for detecting the presence of specific antibodies to a multiplicity of allergens, simultaneously in a single test, or determining the total IgE antibody content in a blood sample, that would not be prone to the occurrence of the hook effect. It also would be advantageous to provide a method of testing blood samples from a patient for allergies that could eliminate the requirement that the patient visit a laboratory or physician's office, as well as the need to have a fairly substantial amount of blood drawn for the purpose of this test. The invention described herein provides such advantages, as well as others that may be apparent from the information described.